Allow searching LIBRARY_PATH with build_library=no

[vilhalmer]

Currently when building a downstream project with build_library=no, the library is still assumed to be present in godot-cpp/bin/ from a previous build. With this change, allow the linker to search the system (LIBRARY_PATH) for it. When build_library=yes, the current behavior of passing the entire path to the linker is retained, since in that case you definitely intended to use the newly-built version.

This change allows more flexibility in the build process of downstream projects: while they still need to have the godot-cpp submodule present for the SConscript, the library can more easily be built as part of a previous step or even preinstalled into the build environment. My specific goal is to save build time in CI with multiple GDExtensions that all use the same version of godot-cpp.

>> scons verbose=yes build_library=no
scons: Reading SConscript files ...
Auto-detected 16 CPU cores available for build parallelism. Using 15 cores by default. You can override it with the -j argument.
Building for architecture x86_64 on platform linux
scons: done reading SConscript files.
…
g++ -o demo/addons/mylib/bin/linux64/libmyliblinux.template_debug.x86_64.so -Wl,-R,'$ORIGIN' -m64 -march=x86-64 -fvisibility=hidden -s -shared -Wl,-rpath=\$ORIGIN src/register_types.os src/mylib.os src/gen/doc_data.gen.os -Lgodot-cpp/bin -lgodot-cpp.linux.template_debug.x86_64
scons: done building targets.

I'm no scons expert so there may be a more idiomatic approach to this. It does technically have a side effect: if a shared library version of godot-cpp somehow existed on the system, it would be preferred because scons has no way to force a given library to be used statically. In practice, this will never happen.

[vilhalmer]

Tweaked so that the compiledb can still be generated with just the downstream project's code.